Methods for determining the charge state and/or the power capacity of charge store

ABSTRACT

Methods are described for determining the state of charge and/or the operability of a charge accumulator using estimates. The information, which is obtained at a least two different operating points or operating conditions of the energy accumulator, is taken into account in the estimates. The estimates are carried out with regard to an instantaneous and/or future state of charge and/or an instantaneous and/or future operability of the charge accumulator. Different methods are executed, depending on the operating point or operating condition. The methods may be run in a processor of a control unit.

BACKGROUND INFORMATION

Different methods for determining the state of charge and operability ofelectric energy accumulators, in particular lead acid batteriescustomary in the automatic industry, are known from the related art. Inmost of the methods, the state of charge of lead acid batteries isdetermined from the open-circuit voltage measured in the idling state,since the open-circuit voltage is proportional to the acid density in abroad range of states of charge (open-circuit voltage method). For thepurpose of estimating the operability or load capacity of the energyaccumulator with regard to a predetermined current consumption or powerconsumption, the internal resistance, which in starter batteries isideally computed from the difference between the measured voltage valuesdivided by the difference between the measured current values during thehigh current load at engine start, is needed in addition to theopen-circuit voltage or the state of charge. A method used fordetermining the battery charge in that manner is known from GermanPublished Patent Application No. 198 47 648 for example.

Continuous information and the state of charge and the operability ofenergy accumulators is required when safety-critical electricalconsumers are used in motor vehicles, e.g., steer-by-wire orbrake-by-wire systems, but also for battery systems and consumermanagement systems, so that the open-circuit voltage and the state ofcharge must also be determined during charging and/or dischargingphases, and the internal resistance also without high current load. Forthis purpose, the state of charge is mostly extrapolated via the currentintegral using charge balancing and the internal resistance is mostlyextrapolated via fixed predefined characteristic curves as a function ofstate of charge and battery temperature. However, during extendedoperation of the energy accumulator without idle phases or high currentload, as well as due to the aging effects not taken into account in thecharacteristic curves, this method results in errors in the estimationof the state of charge and operability.

To prevent these errors, the related art describes model-basedestimation methods which constantly adjust the state variables andparameters of a mathematical model of the energy accumulator to the realstate variables and parameters by continuously measuring voltage,current, and temperature. Such model-based estimation methods are knownfrom German Published Patent Application No. 199 59 019 for example. Inthe known methods, state of charge and operability of the energyaccumulator are calculated from state variables and parameters sodetermined. The disadvantage of these methods is the fact that in orderto cover the entire operating range of the energy accumulator withregard to discharging-/charging current range, state of charge,temperature, as well as aging effects, a complex, and as a rulenon-linear, model of the energy accumulator is required, having manystate variables and parameters to be estimated and which may only beanalyzed at a great expense.

Alternatively simpler models covering only individuals operating pointsof the battery, e.g., only the discharging operation, have advantages;however, they allow an accurate determination of state of charge andoperability only at these operating points. Such simple models aredescribed in German Published Patent Application No. 100 56 969 forexample.

SUMMARY OF THE INVENTION

An object of the present invention is to make the most accuratedetermination of the state of charge and the operability of a chargeaccumulator possible over a large operating range without great expense.By using a weighted correction of the state variables and parametersestimated from at least two methods that are active at two differentoperating points via continuous measurement of voltage, current, andtemperature, the method according to the present invention makes a moreaccurate estimation of the current and future state of charge andoperability of the energy accumulator, in particular a motor vehiclelead battery, possible over a large operating range compared to theindividual methods.

The method according to the present invention combines the advantage ofthe open-circuit voltage methods, i.e., the accurate determination ofthe open-circuit voltage, i.e., the state of charge in phases of thebattery without load and the internal resistance at high current load(e.g., engine start), and the advantage of simple model-based estimationmethods using which open-circuit voltage and internal resistance, aswell as other optional state variables and parameters may be estimated,even during operation without idling or high current loads, thereby,compared to the individual methods, enabling a more accuratedetermination of the state of charge and the operability of the batteryover a large operating range without complex battery models.

For calculating the state of change and the operability, the minimumrequired variables open-circuit voltage and internal assistance, as wellas other optional state variables and parameters, are calculated in anadvantageous manner from the values of the individual methods byweighted correction, their weighting being selected according to theirreliability at the current operating point of the battery.

Predictions of the future operability are possible via extrapolation ofthe currently estimated state variables and parameters for state ofcharge and temperature to a later point in time, so that, the example,the capability of the battery to start a vehicle parked for several daysmay also be estimated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a battery state detection system according to the presentinvention.

FIG. 2 shows a predictor model.

DETAILED DESCRIPTION

FIG. 1 shows the basic structure of the battery state detection systemusing two state estimation and parameter estimation methods active attwo different operating points of the battery. The number of the methodsused is not necessarily limited to two; however, at least one method ismodel-based, i.e., the state variables and parameters of a battery modelare adapted to the real values, e.g., via a recursive least-squareestimator, e.g., an extended Kalman filter.

State variables z(e.g., open-circuit voltage U₀₀) and parameters p(e.g.,internal resistance R_(i)) required for determining the state of chargeand the operability of the battery are obtained from continuousmeasurement of battery voltage U_(Batt), battery current I_(Batt), andbattery temperature T_(Batt) by the state and parameter estimatingsystem. The state of charge calculation determines state of charge socfrom vector z of the state variables and from the instantaneous batterytemperature T_(Batt), while the instantaneous operability of the batteryis estimated via voltage response U_(Batt,pred)(t) of a battery modelinitialized using state vector z and parameter vector p to a predefinedload current profile I_(Load)(t).

If the operability of the battery at a future point in time is ofinterest, e.g., the starting capability is queried after the vehicle wasparked for several days, then instantaneous variables z and parametersp, as well as instantaneous battery temperature T_(Batt) areextrapolated to values z′, p′, and T_(Batt)′ to be expected at thefuture point in time. In order to pre-estimate the reduction in thestate of charge as a function of the parking time of the vehicle, thevariation of closed-circuit current I_(Rest)(t) in the parked vehiclemust be known.

Using method A, here referred to as open-circuit voltage method,open-circuit voltage U₀₀ is determined during no-load phases of thebattery and internal resistance R₁ of the battery is determined during ahigh current load (e.g., engine start). In addition, further variables,e.g., (acid) capacity Q₀ may be derived from measured variables currentI_(Batt), voltage U_(Batt), and temperature T_(Batt), or calculatedvariables U₀₀ and R_(i). The state variables determined by method A arecombined in state vector z _(A), and the parameters are combined invector p _(A). Method B is model-based and also estimates at leastopen-circuit voltage U₀₀ and internal resistance R_(i), however,compared to method A, also in other or additional operating stages ofthe battery (e.g., discharge). The state variables determined by methodB are combined in state vector z _(B), and the parameters are combinedin vector p _(B).

In each calculation step k, state vector z _(k+1) is calculated by usingweighted differences z _(A,K)−z _(k), z _(B,k)−z _(k) and parametervector p _(k+1) is calculated by using weighted differences p _(A,k)−p_(k), p _(B,k)−p _(k) starting with starting values z _(k=0)=z ₀ and p_(k=0)=p ₀:z _(k+1) =z _(k) +G _(z,A)*( z _(A,k) −z _(k))+ G _(z,B)*(z _(B,k) −z_(k))p _(k+1) =p _(k) +G _(p,A)*( p _(A,k) −p _(k))+G _(p,B)*(p _(B,k) −p_(k))

Weighting matrixes G _(z,A), G _(z,B), G _(p,A), and G _(p,B) are squarediagonal matrixes whose main diagonal elements g_(z,A,i=1 . . . n),g_(z,B,i=l . . . n), g_(p,A,j=1 . . .m), g_(z,B,j=l . . . m) specify thedegree of correction of the n state variables and the m parameters andmust fulfill the following requirements, so that the sequences z_(k=0),z _(k=1),z _(k=2) . . . and p _(k=0),p _(k=1),p _(k=2) . . .converge:g _(z,A,i) +g _(z,B,i)≦1, i=1 . . . ng _(p,A,j) +g _(p,B,j)≦1, j=1 . . . m

The weightings are selected in such a way that state variables andparameters which at the instantaneous operating point are moreaccurately determined by using one method than the other, contributemore to the correction. For example, the estimated variables of themodel-based method may flow into the correction only when the estimatingalgorithm has become stable, when the estimated variables are uniquelyidentifiable (observability), and when the battery operates at pointswhich are also described by the underlying model (e.g., discharge). Inall other cases the corresponding weightings must be set g_(z,B,i) andg_(p,Bj)=0.

An example of a particular variant of an embodiment of the battery stateof charge detection for predicting the operability of lead batteries inmotor vehicles is described in the following:

Predictor Model

For estimating the operability of a lead battery under short-time load(on the order of 10 sec) using currents on the order of I_(Load)≦−100A(counting direction I<0A for discharge) as it typically occurs, e.g., inthe operation of electric braking and steering systems, as well as atengine start in motor vehicles. The following simple predictor model,illustrated in FIG. 2, may be used.

Using the equivalent diagram components:

-   -   I_(Load)=predefined current for which operability is to be        tested    -   U₀₀=open-circuit voltage    -   R_(i)=ohmic internal resistance    -   Diode=non-linear resistance of the crossover polarization    -   U_(Ohm)=R_(i)*I_(Load)=ohmic voltage drop at predefined current        profile I_(Load)    -   U_(D)=f(I_(Load), T_(batt))=characteristic curve of the        stationary crossover voltage drop at predefined current profile        I_(Load) and battery temperature T_(batt)

Formula determined from measurements:U _(D)(I _(Load) ,U _(D0))=U _(D0)*1n(I _(Load)/(−1A)),I _(Load)<0Ausing the temperature-dependent crossover parameter:U _(D0)(T _(Batt))=4.97e−7*(T _(Batt)/° C.)³−4.87e−5*(T _(Batt)/°C.)²+1.82e−3*(T _(Batt)/° C.) . . . −131e−1

U_(Batt,pred)=U₀₀+R_(i)*I_(Load)+U_(D)(I_(Load),U_(D0))=predictedvoltage response for battery current I_(Load)

The following prerequisites must be met for the applicability of thepredictor model:

-   -   the discharge due to the predefined load profile I_(Load)(t) is        negligible compared to the battery capacity, i.e., open-circuit        voltage U₀₀ may be assumed to be constant,    -   during the load with L_(Load)(t), the crossover voltage becomes        stabilized at its steady-state final value predefined by        characteristic curve U_(D)=f(I_(Load),T_(batt)), i.e., the load        is applied sufficiently long and is sufficiently high (time        constant of U_(D)˜1/I_(Load)),    -   the concentration overvoltage, not considered in the mode, which        is caused by acid density differences in the battery, is        negligible,    -   charges which are possibly stored in additional capacitances        (e.g., double layer capacitance between electrodes and        electrolyte) outside the actual battery capacity are not        considered (worst case scenario).

These prerequisites are met for the described load in the state ofcharge range of soc>approximately 30% and for battery temperatures ofT_(Batt)>approximately 0° C., as well as soc>approximately 50% andT_(Batt)>approximately 0° C.

State variables and parameters are determined on the basis of thefollowing considerations:

State variable U₀₀, as well as parameters R_(i) and U_(D0) of thepredictor model, are determined by using two different methods:

Method A determines U_(00,A) from measurements of the idling voltage anunloaded battery and R_(i,A) by analyzing the quotient of differences ofthe voltage and current values measured at engine start, while crossoverparameter U_(D0,A) is not estimated by method A but rather calculatedvia the above-mentioned characteristic curve.

In addition, method A determines the battery (acid) capacity from twoopen-circuit voltage determinations U_(00,A1) and U_(00,A2), as well asthe current integral (charge balance)q=∫I_(Batt)(t)dt:Q _(0,A) =q*(U _(00,max)(25° C.)−U _(00,min)(25° C.)/(U _(00,A,2)(25°C.)−U _(00,A,1)(25° C.)where U_(00,max)=open-circuit voltage of the fully charged battery andU_(00,min)=open-circuit voltage of the empty battery at T_(Batt)=25° C.

Using Q_(0,A), current charge balance q_(k), and current batterytemperature T_(Batt,k), method A tracks open-circuit voltage U_(D0,0),determined during the idle phase, during operation in each time step k:U _(00,A,k)(25° C.)=U _(00,A,0)(25° C.)+q _(k) /Q _(0,A)*(U_(00,max)(25° C.)−U _(00,min)(25° C.)U _(00,A,k) =U _(00,A,k)(25° C.)+Tk _(U00)*(T _(Batt,k)−25° C.), Tk_(U00)=1.38e−6V/° C.

Internal resistance R_(i,A,0), determined at the start, is tracked in asimilar manner during operation via a characteristic curve as a functionof current open-circuit voltage U_(00,A,k) and instantaneously measuredbattery temperature T_(Batt,k):R _(i,k) =f(R _(i,A,0) ,U _(00,A,k) ,T _(Batt,k))

By adjusting a suitable battery model in discharge range (I_(Batt)<0A),method B estimates open-circuit voltage U_(00,B), internal resistanceR_(i,B), as well as crossover parameter U_(D0,B), and battery capacityQ_(0,B). The variables needed for determining the state of charge andoperability are calculated from the state variables and parametersdetermined by methods A and B using a weighted correction; a constantsampling rate of 0.01 sec has been assumed for the individual timesteps.U _(00,k+1) =U _(00,k) +g _(U00,A)*(U _(00,A,k) −U _(00,k))+g_(U00,B)*(U_(00,B,k) −U _(00,k))whereU _(00,0) =U _(00,A,0) , g _(U00,A)=1−|q _(k) |/Q ₀ , g _(U00,B) =|q_(k) |/Q ₀i.e., with an increasing absolute value of charge balance |q_(k)|,starting value U_(00,0)=U_(00,A,0) determined by method A from an idlephase is corrected to an increasing degree by value U_(00,B,k)determining by method B during vehicle operation.R _(i,k+1) =R _(i,k) +g _(Ri,A)*(R _(i,A,k) −R _(i,k))+g _(Ri,B)*(R_(i,B,k) −R _(i,k))whereR _(i,0) =R _(i,A,0) , g _(Ri,A)=0, g _(Ri,B)=1.e−3i.e., starting value R_(i,0)=R_(i,A,0) determined by method A at enginestart is corrected during vehicle operation to value R_(i,B,k)determined by method B using constant weighting g_(Ri,B)=1.e−3.U _(D0,k+1) =U _(D0,k) +g _(UD0,A)*(U _(D0,A,k) −U _(D0,k))+g_(UD0.B)*(U _(D0,B,k) −U _(D0,k))whereU _(D0,0) =U _(D0,A,0) , g _(UD0,A)=0, g _(UD0,B)=1.e−3i.e., crossover parameter U_(D0,A) predefined by method A viacharacteristic curve U_(D0)(T_(Batt)) is corrected to value U_(D0,B,k)estimated by method B during vehicle operation using constant weightingg_(UD0,B)=1.e−3.

Capacity Q₀ is not really needed for the prediction of operability;however, value Q_(0,A,0) determined from idle phases by method A may beimproved by values Q_(0,B,k) estimated by method B during vehicleoperation. Since the accuracy of Q_(0,B,k) increases with increasingabsolute value of charge balance |q_(k)|, the weighting was selectedproportional to this valueQ _(0,k+1) =Q _(0,k) +g _(Q0,A)*(Q _(0,A,k) −Q _(0,k))+g _(Q0,B)*(Q_(0,B,k) −Q _(0,k))whereQ _(0,0) =Q _(0,A,0) , g _(Q0,A)=0, g _(Q0,B)=5.e−4*|q _(k) |/Q _(0,k)

Calculation of the Instantaneous State of Charge:

Relative state of charge soc is calculated from instantaneouslydetermined open-circuit voltage U₀₀ (state variable) and instantaneousbattery temperature T_(Batt)(measured variable):soc=(U ₀₀(25° C.)−U _(00,min)(25° C.))/(U _(00,max)(25° C.)−U_(00,min)(25° C.))whereU ₀₀(25° C.)=U ₀₀ =Tk _(U00)*(T _(Batt)−25° C.), Tk _(U00)=1.38e−6V/°C.U _(00,max)(25° C.)=maximum value of the open-circuit voltage at room temperature and fullycharged battery U_(00,min)(25° C.)=minimum value of the open-circuitvoltage at room temperature and empty battery (after removal of chargeQ₀).Calculation of the Instantaneous Operability

The instantaneous operability is determined by battery voltageU_(Batt,pred) under predefined load current I_(Load) calculated by usingthe predictor model, and the instantaneously estimated state variablesand parameters (U₀₀, R_(i), U_(D0)):U _(Batt,pred) =U ₀₀ +R _(i) *I _(Load) +U _(D)(I _(Load) ,U _(D0))

As the absolute measure for the operability of the energy accumulator(SOH=State of Health), the distance of the minimum value of thepredicted battery voltage to a lower limit voltage U_(Batt,limit) atwhich the energy accumulator just about generates the power required forthe considered user (e.g., electric steering and brake systems, starter,. . . ) may be used:SOH _(abs) =min(U _(Batt,pred))−U _(Batt,limit)

The relative measure is obtained by relating SOH_(abs) to the differenceobtained in the most favorable case, i.e., for a new, fully chargedbattery and at high temperatures:SOH_(rel)=(minU _(batt,pred))−U _(batt/limit))/(U _(Batt,pred,max) −U_(batt,limit))Calculation of Future Operability

Future operability may be estimated by inserting the state variables(U₀₀′) and parameters (R_(i)′, U_(D0)′), extrapolated to the futurepoint in time with regard to battery temperature and state of charge,into the prediction equation. Temperature T_(Batt)′ to be expected maybe determined by averaging the battery temperatures over the previous 10to 14 days. For worst case scenarios, 10K are once more subtracted fromthis value.

Open-circuit voltage U₀₀′ to be expected after x days of parking of thevehicle is determined via the drop in the state of charge based on thedischarge due to closed-circuit current I_(Rest):U ₀₀(25° C.)′=U ₀₀(25° C.)+I _(Rest) *x*24h/Q ₀*(U _(00,max)(25° C.)−U_(00,min)(25° C.))U ₀₀ ′=U ₀₀(25° C.)′+Tk _(U00)*(T _(Batt)′−25° C.),Tk _(U00)=1.38e−6V/°C.

Internal resistance R_(i)′ is extrapolated by using characteristic curveR_(i)′=f(R_(i),U₀₀′,T_(Batt)′), while crossover parameter U_(D0)′ iscalculated via characteristic curve U_(D0)(T_(Batt)′).

1. A method for determining at least one of a state of charge and anoperability of a charge accumulator in accordance with an estimate,comprising: taking into account in the estimate information obtained atat least two different operating points of the charge accumulator;executing at least an open-circuit voltage operation and a model-basedestimation operation; and determining the state of charge in accordancewith information obtained in each of the open-circuit voltage operationand the model-based estimation operation.
 2. The method as recited inclaim 1, further comprising: forming a weightable correction variablefrom the information.
 3. A method for determining at least one of astate of charge and an operability of a charge accumulator in accordancewith an estimate, comprising: taking into account in the estimateinformation obtained at at least two different operating points of thecharge accumulator; and performing an estimation in accordance with atleast one of: at least one of an instantaneous state of charge and afuture state of charge, and at least one of an instantaneous operabilityof the charge accumulated and a future operability of the chargeaccumulator.
 4. The method as recited in claim 2, further comprising:operating a predictor to perform an estimation in accordance with atleast one of a future state of charge of the charge accumulator and afuture operability of the charge accumulator.
 5. The method as recitedin claim 1, wherein: two operating states include an idle state of thecharge accumulator and an active state of the charge accumulator.
 6. Themethod as recited in claim 1, further comprising: forming a mathematicalmodel that is processable in accordance with a predefinable variable. 7.A method for determining at least one of a state of charge and anoperability of a charge accumulator in accordance with an estimate,comprising: taking into account in the estimate information obtained atat least two different operating points of the charge accumulator; andimplementing a state estimate and a parameter estimate; wherein anopen-circuit voltage operation and a model-based estimation operationare used for the state estimate and the parameter estimate.
 8. A methodfor determining at least one of a state of charge and an operability ofa charge accumulator in accordance with an estimate, comprising:determining a state variable and a state parameter from the followingmeasured variables: a battery voltage, a battery current, and a batterytemperature according to a first operation; determining an additionalstate variable and an additional sate parameter according to a second,model-based operation; obtaining a correction variable from at least oneof the state variable, the additional state variable, the stateparameter, and the additional state parameter; calculating the state ofcharge in accordance with at least one of the state variable and theadditional state vehicle; and predicting the operability of the chargeaccumulator and determining the battery voltage in accordance with atleast one of the state variable and the additional state variable and atleast one of the state parameter and the additional state parameter. 9.The method as recited in claim 8, wherein: the first operation includesan open-circuit voltage operation.
 10. A device for determining at leastone of a state of charge and an operability of a charge accumulator inaccordance with an estimate, comprising: an arrangement for taking intoaccount in the estimate information obtained at at least two differentoperating points of the charge accumulator to perform at least anopen-circuit voltage operation and a model-based estimation operation,and to determine the state of charge in accordance with informationobtained in each of the open-circuit voltage operation and themodel-based estimation operation.
 11. The device as recited in claim 10,further comprising: an arrangement for estimating a state variable and astate parameter; an arrangement for calculating the state of charge; andan arrangement for predicting the operability, wherein: the arrangementfor estimating, the arrangement for calculating, and the arrangement forpredicting are connected to one another.
 12. A device for determining atleast one of a state of charge and an operability of a chargeaccumulator in accordance with an estimate, comprising: a determiningarrangement to determine a state variable and a state parameter from atleast one of a battery voltage, a battery current, and a batterytemperature according to a first operation, and to determine anadditional state variable and an additional state parameter according toa second, model-based operation; an obtaining arrangement to obtain acorrection variable from at least one of the state variable, theadditional state variable, the state parameter, and the additional stateparameter; a determining and predicting arrangement to determine thestate of charge in accordance with at least one of the state variableand the additional state variable, to predict the operability of thecharge accumulator, and to determine the battery voltage in accordancewith at least one of the state variable and the additional statevariable and at least one of the state parameter and the additionalstate parameter.
 13. The device as recited in claim 12, wherein thefirst operation includes an open-circuit voltage operation.